EVOLUTIONARY INDEPENDENCE OF MORPHOLOGICAL FEATURES AND VARIABILITY IN POPULATIONS OF MICRASTER (ECHINOIDEA, SPATANGOIDEA) FROM THE UPPER CRETACEOUS OF PICARDY (NW FRANCE)

Morphological variation in Turonian and Coniacian Micraster (Irregular Echinoids) from Picardy is analyzed in samples from 4 beds stratigraphically superimposed in the chalk cliff and numbered C2, C3, C4 and C5. The statistical study is made of 23 characters (12 quantitative and 11 qualitative). Uni- and multivariate analyses, complemented by observations of populations, have shown M. leskei (Des Moulins) to be in the 2 lower beds and M. decipiens (Bayle) in the upper ones. The growth of M. leskei is analyzed. The biometrical study (bi- and multivariate analyses) of growth processes of the C2-bed urchins (which are small) and of the C-3 bed urchins (which are sometimes much bigger) shows the homology of their ontogeny and confirms the name large M. leskei given by R.B. Stokes for the C3 urchins. The genetic or ecological origin of that variation is discussed. The analysis (multivariate) demonstrates the considerable variability of M. decipiens. This species contains several morphological types: trapezoidal or heart-shaped urchins, and conical-shaped urchins. Gibbous forms are simple variants and the 2 1st forms may be considered as morphs of the same species. It is possible that these forms are separate species.